1. Technical Field
The present invention relates in general to improved semiconductor wafer processing, and in particular to improving flatness between a semiconductor wafer and a vacuum chuck. Still more particularly, the present invention relates to an improved apparatus and method for applying a vacuum to a semiconductor wafer through a vacuum chuck to improve flatness of the wafer during processing.
2. Description of the Related Art
As semiconductor or giant magneto resistive (GMR) head wafers are processed, various metallic and non-metallic layers are deposited on one side of the original wafer substrate. Even if the original substrate is very flat, the deposition of material layers with residual tensile or compressive stresses on only one side of the wafer will cause the entire wafer to distort from its original state of flatness in directions normal to the plane of the wafer. Because of this distortion, there is a need for the wafer to be held and restrained as flat as possible during the various process steps, such as photoresist application and exposure, material deposition, and metrology. Photoresist exposure processing is particularly sensitive to flatness since non-flat deviations can lead to location errors of the features being printed on the wafer, and oversized or undersized feature printing.
Prior art process machines utilize a vacuum chuck to hold the wafer flat during processing. A vacuum chuck is a platform that may be larger, smaller, or the same size as the wafer. The wafer is placed on the chuck by an operator or a robot with the non-processed side of the wafer facing downward against the chuck. Typically, the surface of the chuck is held to an extremely tight flatness tolerance, although non-flat shapes such as spherical or cylindrical shapes also can be used. The face of the vacuum chuck facing the wafer usually has many open holes through which a vacuum is applied. The vacuum originates from an outside vacuum source via tubes or tubular ports inside the chuck. During processing, the vacuum holds the wafer in place on the chuck. After processing, the vacuum is discontinued so that the wafer may be removed from the chuck.
Most prior art process machines apply the vacuum to all of the holes in the face of the chuck at the same time. For example, FIG. 1 depicts a flat, circular vacuum chuck 11 with a plurality of vacuum holes 13 arrayed across its surface. The small numerals located adjacent to the lower right side of each of the holes 13, schematically illustrate the timing sequence of the vacuum applied to the holes 13. In FIG. 1, since each of the holes 13 has the same numeral xe2x80x9c1xe2x80x9d, the vacuum is applied to each of the holes 13 at the same time.
However, since neither the wafers nor the vacuum chucks are perfectly flat, a problem arises with this prior art method as the vacuum is applied simultaneously across all of the holes. The localized areas on the wafer where the gaps between the wafer and the chuck are smallest will adhere to the chuck first. Other areas on the wafer, where the gaps between the wafer and the chuck are larger, will not be pulled tightly against the chuck due to static friction between the wafer and the chuck at the earlier adhesion areas. As a result, tiny gaps remain between the wafer and the chuck such that the wafer does not completely deform to the flat shape of the chuck. These gaps of separation can be even larger when the wafer substrates are made from rigid materials such as the titanium carbide used to manufacture GMR heads.
U.S. Pat. No. 5,094,536 discloses a deformable vacuum chuck with distorting actuators that may be manipulated to directly compensate for distortions in a wafer. This is a very elaborate and complicated mechanical apparatus that requires feedback from an interferometer system. U.S. Pat. No. 5,564,682 discloses a sequenced vacuum method for correcting symmetrical, single-plane wafer distortions around a line that is perpendicular to the wafer and through its center. Unfortunately, this reference has very limited application since many wafer distortions are asymmetrical and/or exist in multiple planes. Thus, an improved apparatus and method for improving flatness between a semiconductor wafer and vacuum chuck is needed.
A flat vacuum chuck for restraining semiconductor wafer substrates and the like during processing applies a vacuum through holes in the chuck in a timed sequence. The localized areas of the wafer adjacent to each of the holes adhere to the chuck in a pattern that is controlled in such a way so as to minimize any residual gaps therebetween. In one application, the vacuum sequencing pattern is analogous to smoothing a curled sheet of paper on a flat surface with both hands by starting at or near the center of the sheet and moving both hands outward along the surface until the sheet is flat. The vacuum may be applied to substrates to overcome all types of distortions, including symmetrical, asymmetrical, and multi-plane distortions, depending upon the orientation of the net internal stresses resulting from the layers deposited on the wafer. The sequenced application of the vacuum through the chuck holes can be timed by installing a solenoid valve on each of the vacuum tubes connected to the ports. Alternatively, the vacuum applied through the chuck holes can be controlled by a single valve via different length tubes between the valve and the surface of the chuck. The holes with the shorter tubes reach nominal vacuum pressure sooner than those with longer tubes.
Accordingly, it is an object of the present invention to provide improved semiconductor wafer processing.
It is an additional object of the present invention to provide improved flatness between a semiconductor wafer and a vacuum chuck.
Still another object of the present invention is to provide an improved apparatus and method for applying a vacuum to a semiconductor wafer through a vacuum chuck to improve flatness of the wafer during processing.
The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the preferred embodiment of the present invention, taken in conjunction with the appended claims and the accompanying drawings.